1. Field of the Invention
The present invention concerns a magnetic bearing for active magnetic centering of a body movable relative to a static body with respect to at least one centering axis perpendicular to a reference axis.
2. Description of the Prior Art
There have already been proposed magnetic bearings and magnetic suspension devices adapted to be integrated into systems comprising parts in relative movement between which there is to be no contact.
The relative movement is usually a rotation, but can also be movement in translation. Generally speaking, the known bearings comprise identical centering means with respect to two axes perpendicular to an axis of rotation.
The magnetic suspension devices are particularly recommended for rotating systems requiring:
a high speed of rotation,
a high level of reliability,
a high degree of precision in terms of positioning in the plane perpendicular to the rotation axis,
a highly stable speed of rotation, and
low energy consumption.
It has been known for some considerable time (EARNSHAW, 1842) that it is impossible to obtain a stable magnetic suspension using only static fields: it is therefore necessary to associate with static magnetic fields (permanent magnets) a minimum of energy (electrical, electrostatic or otherwise) to enable a body to be suspended.
In 1974 the assignees of the inventors developed a magnetic suspension device in which a floating rotating body is having degree of freedom controlled by a servomechanism whereas its other degrees of freedom are stabilized by permanent magnets (French patents 74 00190 and 74 40556, equivalent to U.S. Pat. No. 3,955,858 to Poubeau).
These systems were decoupled systems. Subsequent work has led to the production of integrated and therefore simplified systems resulting in increased reliability.
Thus there are known several magnetic suspension devices having two degrees of freedom controlled actively and three degrees of freedom controlled passively. They relate to the suspension of rotors for which there is usually a requirement for a high mass and therefore a high inertia.
U.S. Pat. No. 4,285,553 (ROBINSON) proposes a system in which the rotor carries a ring of magnets and the stator comprises flat internal pole rings linked by a cylindrical ferromagnetic wall. Electrical coils are placed on this latter cylindrical wall to avoid discontinuity of the polepieces and to minimize Eddy currents. The lines of magnetic force created by a pair of diametrally opposed coils energized simultaneously circulate in the pole rings of the rotor over one-half the perimeter and are closed via the cylindrical wall of the stator in two diametrally opposed areas facing the coils. These field lines therefore cross the airgap between the polepieces four times, leading to high losses. Certain of the lines of magnetic force are closed after covering only one quarter of the perimeter, passing through the airgaps of another pair of coils designed to control a second centering axis, which leads to interaction between the control axes that is difficult to control in order to achieve stability of the system.
U.S. Pat. No. 4,077,678 (STUDER) describes a flywheel energy storage device in which the magnetic type suspension groups together on the stator passive axial centering means and active radial centering means. Permanent magnets substituted for the cores of the coils result in a significant reduction in the efficiency of the servomechanisms. To increase the axial and radial stiffness due to the magnets by increasing their volume it is necessary to increase the volume of the coils, the turns of which facing the magnet are nevertheless totally ineffective, since their core is a permanent magnet.
U.S. Pat. No. 4,043,614 (LYMAN) describes a device in which the stator comprises a thick disk magnetized perpendicular to the rotation axis and clamped axially between two pole disks the diameter of which is slightly greater than that of the disk so that they can receive at their periphery two pairs of diametrally opposed electrical coils. The rotor, disposed around the stator, comprises a ring magnetized axially in the opposite direction to the disk and clamped between two flat pole rings. The principal disadvantage of this device is related to the absence of a magnetic flux permeable path between the polepieces carrying the coils, which significantly limits their efficiency.
U.S. Pat. No. 4,294,493 ((SINDLINGER) describes a magnetic suspension device which has the same disadvantages as that described in U.S. Pat. No. 4,043,614. The windings are very large, resulting in a considerable stator mass as compared with the rotor.
French patent 2 524 090 filed by the assignees of the inventors overcomes the disadvantages of the previous patents. It proposes a device in which, encountering only the reluctance of two airgaps, the lines of magnetic force follow a very short path situated in a plane containing the axis of rotation. The short length of this path reduces losses. Also, no unwanted interaction between the pairs of sectors associated with different axes can occur. In this suspension, however, the magnetically suspended part (rotor) comprises a permanent magnet, which removes any possibility of very high rotation speeds as these would be prejudicial to the mechanical durability of the magnet ring. Moreover, applications in which the rotating body is situated in a low pressure or corrosive gas are difficult to implement. Note also that the pole circuit of the coils includes discontinuities the consequence of which is rotational losses due to the Eddy currents which are therefore generated.
U.S. Pat. No. 4,000,929 (STUDER) describes a magnetic bearing for suspending a ring; this bearing comprises a central magnet magnetized axially clamped between ferromagnetic plates extended on crossed diameters by flanges around which coils are wound and the extreme edges of which define with the ring a single airgap. The magnetic flux in one pair of flanges is closed via the other pair of flanges, which procures radial centering of the ring. A bearing of this kind, extremely difficult to implement, has the major disadvantage that its mode of operation is based on coupling between the two control axes.
An object of the invention is to alleviate the aforementioned disadvantages.